A promising alternative to gas-phase deposition of 2D semiconductors is solution-based synthetic strategies. These low-temperature methods yield colloidal “inks” of liquid-dispersed 2D nanostructures that allow for the straightforward fabrication of flexible devices. However, the colloidal nature of these 2D semiconductors introduces new challenges to the characterization of their materials properties. Here, we employ two distinct optical spectroscopic techniques to extract structural and electrical information from these 2D nanocrystals. First, bottom-up solution routes were applied to produce nearly monodisperse colloidal 2D SnS semiconductors. Next, their crystallographic phase and structure was investigated using Raman spectroscopy, where we probed the anisotropic, black phosphorus-like bonding arrangement found within the layers of SnS. Finally, we interrogate their electrical transport properties using a novel time-resolved terahertz spectroscopy strategy that allows for the non-contact determination of photoconductivity and carrier mobility within individual 2D colloidal nanocrystals. We believe these metrological innovations will prove applicable and of paramount importance to the characterization of other solution-synthesized semiconductor nanocrystal systems.

Black phosphorus (BP) is an emerging two-dimensional semiconducting material with great potential for nanoelectronic and nanophotonic applications. Many theoretical studies have predicted the anisotropic optical properties of BP, but the direct experimental quantification remains challenging since the ease of BP’s degradation and the indirect nature of conventional approaches. This work reports a direct investigation of the birefringent optical constants of micrometer-thick BP samples with picosecond (ps) interferometry. In this ps-interferometry approach, an ultrathin (~5 nm) platinum layer is deposited for launching acoustic waves, which also naturally protects the BP flake from degradation. The birefringent optical constants of BP for light polarization along the two primary crystalline orientations, zigzag and armchair, are directly obtained via fitting the attenuated Brillouin scattering signals. A bi-exponential model is proposed to analyze the BS signals for a random incident light polarization. The BP experimental results and the associated measurement sensitivity analysis demonstrate the reliability and accuracy of the ps-interferometry approach for capturing the polarization-dependent optical properties of birefringent materials.

Magneto-optic Kerr effect (MOKE) with micrometer-scale spatial resolution opens up an avenue to study the magnetic structures in layered materials. In this talk, we report a variable temperature Kerr microscope housed in a room temperature bore superconducting magnet with the vector magnetic field up to 2 T in any direction. Because the sample and magnet are cooled separately by closed cycle cryocoolers, the sample temperature and magnetic field could be controlled independently. Thus, the sample temperature could be varied continuously from 10 K to 300 K or more. By employing the photoelastic modulation technique, the Kerr angle resolution is better than 0.5 mRad at a tunable wavelength. While the spatial resolution is smaller than 3 um by using an aspherical lens, the scan area could reach to 120 x 120 um2 with the aid of a 2D Galvo-Mirrors. To demonstrate its powerful performance, we will show experimental results in mechanically exfoliated few-layer magnetic materials such as chromium trihalides.

Bulk chromium triiodide (CrI3) has long been known as a layered van der Waals ferromagnet. However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet, providing a new platform for investigating light-matter interactions and magneto-optical phenomena in the atomically thin limit. Here, we report spontaneous circularly polarized photoluminescence in monolayer CrI3 under linearly polarized excitation, with helicity determined by the monolayer magnetization direction. In contrast, the bilayer CrI3 photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI3 bilayers. Distinct from the Wannier-Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors, our absorption and layer-dependent photoluminescence measurements reveal the importance of ligand-field and charge-transfer transitions to the optoelectronic response of atomically thin CrI3. We attribute the photoluminescence to a parity-forbidden d-d transition characteristic of Cr3+ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized molecular orbital nature.

Two-dimensional materials have demonstrated interesting nonlinear optical (NLO) properties. Despite extensive researches, little attention has been paid to the relationship between the nonlinear optical strength of 2D materials and the properties of the substrates they are sitting on. In this work, we discovered that the strength of nonlinear optical signal depends significantly on the SiO2 thickness in the SiO2/Si substrates. A 40-fold increase of graphene nonlinear photoluminescence (NPL) signal was observed when the SiO2 thickness was varied from 270 nm to 125 nm under 800nm excitation. Furthermore, we have extended our measurements to include transient absorption (TA) and second harmonic generation (SHG) of graphene and MoS2, confirming that SiO2 thickness has similar effects on all of the three major types of NLO signals. Our observations provide a critical reference for NLO researches of two-dimensional materials, and our model may serve as a quick guidance for choosing the optimum substrates to conduct nonlinear optics and optoelectronic studies.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising 2D materials with interesting optoelectronic, catalytic and sensing applications whose nanoscale optical characterization provides detailed structure-function information which is a challenge for typical far-field diffraction-limited techniques. We investigate monolayer and few-layer 2D materials and heterostructures using tip-enhanced Raman scattering (TERS) and tip-enhanced photoluminescence (TEPL) with a few nanometer spatial resolution. We investigate the limits of signal enhancement by varying the tip-sample gap and reveal quantum plasmonic quenching for sub-nanometer gaps. Quantum plasmonics provides a new regime for the generation of excitons and trions in 2D materials. We present examples of near field control of optical signals from excitons and trions, and investigate various enhancement mechanisms.

Two-dimensional monolayer semiconductors, e.g. transition metal dichalcogenides, have giant second order nonlinearity due to the intrinsic lack of inversion symmetry. However, the sub-nanometer thickness of monolayer limits the conversion efficiency and hence/consequently the potential application. Here, we experimentally show that the second-harmonic generation (SHG) of WS2 can be enhanced by integrating it on a plasmonic metasurface. The SHG enhancement factor of 400 can be realized due to the strong field confinement of the plasmonic dark mode. Meanwhile, the polarization dependence of SHG can also be controlled by the plasmonic mode. An optical information encoding method is demonstrated based on this property. Our results suggest that the planar hybrid structure made by transition metal dichalcogenide and plasmonic metasurface provides an integrated platform for nonlinear optics frequency conversion to generate on-chip tunable light source.

Time- and angle-resolved photoemission (tr-ARPES) is a powerful technique that measures the transient dynamics of band structure in various condensed matter systems. In a tr-ARPES setup, one laser pulse is used to pump the system to an excited state, and a subsequent ultraviolet pulse is used to probe the photoemitted electrons at different time delays after the arrival of the pump pulse. Most tr-ARPES measurements are carried out with low energy (6-7 eV) photons, limiting the measurement range of the momentum space. In other approaches, 20-30 eV photons through high-harmonic generation (HHG) processes are used at the cost of a worse energy resolution (> 70 meV) due to the large bandwidth of the HHG pulses. Here we report tr-ARPES measurements with time and energy resolutions of 250 fs and 16 meV, respectively, using intermediate 11 eV laser pulses. Extreme ultraviolet pulses were produced through a third harmonic generation of 346 nm (3.58 eV) femtosecond laser pulses in Xe gas. We will show tr-ARPES measurements taken on single crystals of a topological insulator Bi2Se3 and a two-gap charge density wave material ErTe3. Our results will allow study of various materials that have interesting phenomena far away from the center of the Brillouin zone.

Dipolar excitons in coupled quantum wells (CQW) offer a unique test bed for studying collective effects of a quantum degenerate system. We have recently studied the behavior of indirect excitons in this system (GaAs/AlGaAs CQW) and found an abrupt gas-liquid phase transition at a critical temperature and excitation power.In this work, we study this transition by Resonant Rayleigh scattering measurements, known to be an insightful probe of critical phenomena. The RRS signal is expected to be strongly enhanced at the exciton resonance; with its linewidth being a measure of disorder in the sample. Here we perform the measurements in a pump–probe configuration: we create the carriers by the laser diode pump and measure the scattered intensity of the weak Ti:Sapphire probe, tuned to the NW exciton resonance. We find that in the liquid phase, at temperatures lower than 1K, the signal becomes symmetric with a significantly narrower width as compared to the signal obtained for the gas. The homogeneous nature and the narrow linewidth suggests that the disorder potential in the sample is effectively screened at these temperatures. This screening is predicted theoretically and shown to be a precursor of superfluidity.